The invention pertains to a method for powdering printed sheets, wherein the powder is removed from a supply container and mixed with air in a mixing device, the powder is distributed by an air stream and applied onto the sheet by a nozzle unit that, if so required, can be adapted to the width of the sheet, the number of outlet openings or the outlet cross section, in particular, of said nozzle unit is changed correspondingly, and the air stream is generated by an air pressure generator. The invention also pertains to a device with a supply container for the powder, a mixing device, in which the powder is mixed with the air stream, an air pressure generator for generating the air stream, and a nozzle unit, from which the air stream containing the powder is discharged and blown onto the sheet, wherein the nozzle unit contains several outlet openings or an outlet cross section that allows the nozzle unit, if so required, to be adapted to the width of the sheet.
It is known, for example, that paper sheets printed by a printing machine are stacked by means of a sheet stacking device. Before the stacking process, the sheets are powdered such that the still moist printing color does not smear during the stacking process. A powdering device, in which powder is mixed with an air stream, is used for powdering the sheets, and the air stream containing the powder is blown onto the surface of the sheet. A nozzle unit that, for example, is mounted on a nozzle beam that extends over the entire width of the sheet transverse to the transport direction is used for this purpose. In order to maintain the powder loss as well as the soiling of the sheet stacking device and the printing machine at a minimum, the active region of the nozzle unit is adapted to the width of the sheet. This is realized by switching on or off individual nozzles or by adapting the outlet cross section to the width of the sheet. For example, if nozzles are switched off, i.e., if the outlet openings of these nozzles are closed, the pressure drop between the blower that generates the air stream and the outlet openings changes. The compressed air made available by the blower now must be discharged through a smaller number of nozzles and a respectively smaller outlet cross section, whereby the flow speed as well as the pressure gradient change so that the air emerging from the nozzles is discharged with a higher speed. Due to the lower pressure drop that occurs if a smaller quantity of outlet nozzles remains open, less powder is stirred up in the mixing device that is constructed in the form of a turbulence chamber, so that less powder is concentrated and, thus, distributed in the air. The specific powder volume which is distributed for a narrow sheet consequently is lower than that for a wider sheet. In addition, the quantity of powder required is highly dependent on the transport speed of the printed paper sheets.
The invention provides a method and/or a device by means of which the specific powder volume admixed in the air stream and, consequently, the distributed powder volume can be adapted to the instantaneous requirements of the sheet in a superior fashion. In a method of the initially mentioned type, this is attained according to the invention due to the fact that the performance of the air pressure generator can be changed, in particular, continuously during the operation.
According to one aspect, the performance of the air pressure generator is adapted to the machine speed or the transport speed of the paper sheets. The method according to the invention essentially provides the advantage that the distributed quantity of powder can be exactly adapted to the requirements of the paper sheet. Consequently, compromises with respect to a slow operating speed and a normal operating speed are eliminated. When starting the operation or reducing the speed, the powder quantity is automatically reduced.
According to other aspects of the invention, the performance of the air pressure generator can also be adapted to other ambient conditions, if so required. These conditions pertain, for example, to the printing material, the size of the printed surface, the printed image, the type of powder, the type of paper, the relative humidity and the temperature. Suitable sensors are provided for this purpose. These sensors forward the measured data, in particular, control currents, to a control unit of the air pressure generator.
According to another refinement, the power of the air pressure generator is adapted to the number of open outlet openings or to the outlet cross section. Due to this measure, wide sheets that, for example, are powdered by 12 nozzles are provided with the air quantity required for this purpose, and a lesser quantity of air is supplied for narrower sheets (e.g., with a half-format) that, for example, are powdered by six nozzles. Consequently, the air volume discharged per nozzle remains the same. This also ensures that the quantity of powder per nozzle remains the same. This means that the performance of the air pressure generator is reduced if the number of outlet openings or the outlet cross section is reduced. In this case, the performance of the air pressure generator can be linearly adapted.
According to yet another refinement, the powder is removed from the supply container by means of a fixed metering device, e.g., a drum metering device or a cell metering device. The exact quantity of powder required for powdering the respective sheet can be withdrawn by the fixed metering device. If one or more nozzles are switched off for powdering narrower sheets, the quantity metered by the fixed metering device is correspondingly reduced. According to a refinement, the pressure of the air stream is adjusted to a value between 0.1 bar and 0.5 bar, in particular, 0.3 bar. A relatively large air volume provided by the air pressure generator corresponds to this low pressure. Consequently, the pressure of the air stream can be maintained at a constant value independently of the number of open outlet nozzles.
According to one preferred aspect of the invention, a side channel compressor is used as the air pressure generator. This compressor requires no maintenance and is not sensitive to dust. In conventional powdering devices, rotary vane compressors are used. However, these compressors require regular and relatively frequent maintenance. In addition, the vanes must be exchanged relatively often. Another problem is that rotary vane compressors must run dry because the air must be free of oil, and, thus, wear is promoted. Rotary vane compressors of this type also generate a relatively high pressure of approximately 1 bar and only a small volume. Rotary vane compressors can only be operated within a very narrow speed range, where this speed range of 2860 l/min to 3430 l/min corresponds to a frequency of 50-60 Hz. At low rotational speeds, the pressing force of the vanes is low resulting in an insufficient seal. At higher rotational speeds, the pressing force becomes excessively high due to the centrifugal force such that the wear increases impermissibly.
The power of the side channel compressor is preferably controlled or regulated with the aid of a frequency converter. Due to this measure, the rotational speed of the compressor and consequently its performance can be adapted relatively easily to the respective requirements. The frequency can be adjusted, for example, between 0 and 100 Hz such that a rotational speed range between 0 and 6000 l/min is achieved.
According to one refinement, an injector is used as the mixing device. In this injector, the air stream is realized in the form of an enveloping air jet, into which the powder is attracted by suction. This provides the significant advantage in that the suction tube can be arranged vertically and the powder must merely drop from the metering device due to gravitational force in order to be transported into the enveloping air jet. Consequently, the transport direction of the powder does not have to be deflected and, thus, no transport medium is required since, as mentioned above, the gravitational force suffices for transporting the powder into the air stream.
According to the invention, the compressor can be controlled by a sensor. This sensor determines the speed of the paper web or another component that is related to the speed of the paper web, e.g., the rotational speed of a driving motor or the like. The quantity of powder being withdrawn can be controlled exactly in this fashion.
The sensor determines, for example, the number of open outlet openings or the outlet cross section and controls the compressor drive correspondingly. However, sensors which determine the volume flow or the pressure in the air stream could also be used. For example, the compressor can be adjusted by means of a pressure sensor in such a way that a uniform pressure always exists in the powder/air stream. However, it would also be conceivable to provide sensors for determining the quantity of the powder contained in the air stream. The abovementioned sensors may be provided alternatively or simultaneously. In addition, it would also be conceivable to provide sensors which determine the printing material, the size of the printed surface, the printed image, the powder type, the paper type, the relative humidity, and the temperature. In this case, the performance of the air pressure generator is varied depending on the sensor data. The paper sheet is always powdered with an optimum amount of powder in this fashion.
The device according to the invention contains an injector in which the air stream is used as the driving air. The driving air is conveyed around a suction line, into which surrounding air is attracted by suction and the powder is attracted by suction with this ambient air. This suction line can be directly connected to the fixed metering device such that the conveyed powder is entrained in the surrounding air. According to the invention, the fixed metering device may consist of a metering drum that is described, e.g., in patent application serial nos. DE 199 37 090.7 and DE 199 37 557.7